1. Field of Invention
The present invention relates to a power converter, and more particularly, the present invention relates to a transistor gate driver with a charge-pump circuit of a controller for the power converter.
2. Description of Related Art
FIG. 1 shows a circuit diagram of a primary side regulated power converter with the constant current output. The power converter (such as a power adaptor) is required to charge a battery with a constant current. The power converter includes a transformer 110 for generating an output (an output voltage VO and an output current IO). The transformer 110 has a primary winding NP, a secondary winding NS, and an auxiliary winding NA. The secondary winding NS generates the output voltage VO and the output current IO through a diode 140 and a capacitor 145. The output voltage VO and the output current IO are generated at an output terminal of the power converter. A first terminal of the primary winding NP is coupled to an input voltage VIN. A second terminal of the primary winding NP is coupled to a drain of a power transistor 120.
A controller 50 generates a gate-drive signal SG. The gate-drive signal SG is coupled to a gate of the power transistor 120 to drive the power transistor 120 for switching the transformer 110 and regulating the output voltage VO and the output current IO of the power converter. In other words, the transformer 110 is magnetized when the power transistor 120 is turned on.
A current-sense device, such as a resistor 125, is coupled between a source of the power transistor 120 and a ground. The resistor 125 detects a switching current IT of the transformer 110 to generate a current-sense signal VCS coupled to the controller 50. The current-sense signal VCS is correlated to the switching current IT.
An anode of the diode 140 is coupled to a first terminal of the secondary winding NS. The capacitor 145 is coupled between a cathode of the diode 140 and a second terminal of the secondary winding NS. The capacitor 145 is further coupled to the output terminal of the power converter.
An anode of a diode 130 is coupled to a first terminal of the auxiliary winding NA of the transformer 110. A second terminal of the auxiliary winding NA is coupled to the ground. A capacitor 135 is coupled between a cathode of the diode 130 and the ground. Once the power transistor 120 is turned off, the secondary winding NS of the transformer 110 will generate the output voltage VO and the output current IO via the diode 140 and the capacitor 145. Meanwhile, through the diode 130 and the capacitor 135, the auxiliary winding NA of the transformer 110 will generate a source voltage VDD coupled to supply the power for the controller 50. The diodes 130 and 140 are used as the rectifier.
Because of the secondary winding NS and the auxiliary winding NA are proportional to the primary winding NP, the current-sense signal VCS and the source voltage VDD are correlated to the output current IO and the output voltage VO respectively. The source voltage VDD is correlated to the output voltage VO according to the following equation (1).
                              V          DD                =                                            T              NA                                      T              NS                                ×                      V            O                                              (        1        )                            where “TNA/TNS” is the turn ratio of the auxiliary winding NA and the secondary winding NS of the transformer 110.        
A voltage divider including resistors 131 and 133 is coupled between the auxiliary winding NA of the transformer 110 and the ground. Through the auxiliary winding NA of the transformer 110, a reflected signal VS coupled to the controller 50 is generated at the join of the resistors 131 and 133. Because of the auxiliary winding NA is related to the secondary winding NS, the reflected signal VS is related to the output voltage VO and the output current IO. The gate-drive signal SG is thus generated in accordance with the reflected signal VS for regulating the output voltage VO and the output current IO of the power converter.
The primary side regulation technologies had been disclosed in many prior arts such as, “PWM controller regulating output voltage and output current in primary side”, U.S. Pat. No. 6,721,192; “Primary-side controlled flyback power converter”, U.S. Pat. No. 6,853,563; “Control circuit for controlling output current at the primary side of a power converter”, U.S. Pat. No. 6,977,824.
FIG. 2 shows the characteristic curve for the output voltage VO versus the output current IO of the power converter shown in FIG. 1. When the output current IO of the power converter reaches the constant current level, the output voltage VO will be decreased for regulating its output current IO as a constant. Once the output voltage VO decreases to a low voltage as shown in a curve 150, the source voltage VDD of the controller 50 will be decreased accordantly (as shown in equation 1). The low source voltage VDD would generate a low voltage gate-drive signal SG that cannot turn on the power transistor 120.
The object of the present invention is to solve this problem. A transistor gate driver is developed to fully turn on the power transistor 120 even when the source voltage VDD is low. A prior art “Transistor drive circuit of power converter operating in a wide voltage range”, U.S. Pat. No. 7,471,121 was designed to solve this problem, but its drawbacks are bigger die size and the higher cost of the circuit of the controller.